Differences in the secretion of the vascular regulators endothelin-1 (ET-1, prostacyclin (PGI2) and thromboxane A2 (TXA2) associated with ageing were investigated in cultured endothelial cells isolated from normal human umbilical veins (HUVECS). HUVECS at different population doubling levels (PDLs) were cultured in medium MCDB-104 supplemented with FBS and ECGF. Cell saturation density was determined by a Coulter counter, and concentrations of ET-1, PGI2 and TXA2 in the media were determined by radioimmunoassay. The cellular and nuclear size of HUVECS increased with advancing age, and the dividing ability decreased. Cell saturation density of HUVECS decreased 5-fold between PDLs 7 and 67 (P < 0.01). The secretion of ET-1 by HUVECS at a young stage of growth (PDL 7.6) increased linearly between 0 and 36 h of incubation (P < 0.01). ET-1 secretion increased approximately 3-fold between PDLs and 67 (P < 0.01). PGI2 secretion increased 6-fold between PDLs 7 and 67 (P < 0.01), and TXA2 secretion increased 18-fold between PDLs 7 and 67 (P < 0.01). The ratio of PGI2 to TXA2 secretion decreased 3-fold between PDLs 7 and 40 (P < 0.01), and remained at the lower ratio between PDLs 40 and 67. This data indicates that the anti-thrombotic or anti-vasoconstrictive role of endothelial cells may decrease during in vitro ageing.